1. Field of the Invention
The present invention relates to a digital signal processing apparatus and, more particularly, to an apparatus adapted for use in recording, reproducing and transmitting an audio signal or the like with a high quality.
2. Description of the Prior Art
In the conventional digital signal processing apparatus, there are known some examples employing the technique of adaptive predictive coding (APC) which encodes an audio signal for compression of information, hence realizing high-efficiency transmission of the audio signal while preventing deterioration of the signal-to-noise ratio and the articulation, as disclosed in Japanese Patent Laid-open Nos. 59-223033, 60-223034, 61-158217 and 61-158218.
In such digital signal processing apparatus, the spectrum of re-quantization noise is selectively switched by the technique of noise shaping to improve the aural signal-to-quantization noise ratio (SNR). (IEEE Transaction on Acoustics, Speech, and Signal Processing, Vol. ASSP-27, No. 3, June 1979; Journal of Electronic Data Communication Society, April 1987, Vol. 70, No. 4, pp. 392-400; Japanese Patent Laid-open Nos. 59-223032, 60-103746 and 61-158220)
FIG. 1 shows a conventional digital signal processing apparatus 1, wherein an input digital signal S.sub.I is fed to a predictive filter 3.
The predictive filter 3 consists of a quadratic filter circuit, and its output signal is supplied to an adder 7 so that a residual signal S.sub.Z1, which corresponds to the difference between such output signal from the filter 3 and the input digital signal S.sub.I, is fed to a maximum detector 8.
The predictive filter 3 is so formed that the frequency characteristic thereof is selectively switched in accordance with a detection output signal Sp obtained from the maximum detector 8, whereby the frequency characteristic H(z) of the residual signal S.sub.Z1 to the input signal S.sub.I is corrected as expressed by a selected one of the following equations: EQU H (z)=1 (1) EQU H (z)=1-0.9375 z.sup.-1 ( 2) EQU H (z)=1-1.796875 z.sup.-1 +0.8125 z.sup.-2 ( 3) EQU H (z)=1-1.53125 z.sup.-1 +0.859375 z.sup.-2 ( 4)
In the maximum detector 8, the detection output signal Sp is produced for each predetermined segmental period on the basis of the residual signal S.sub.Z1 and is fed to the predictive filter 3, whereby the frequency characteristic of the predictive filter 3 is selectively switched in such a manner as to lower the level of the residual signal S.sub.Z1 outputted during each predetermined segmental period.
Thus, in case the spectrum of the input digital signal S.sub.I is distributed in a higher frequency band, the frequency characteristic represented by Eq. (1) is selected. And when the spectrum is thereby more flattened, the frequency characteristics represented by Eqs. (2), (3) and (4) are sequentially selected. In this manner, the predictive filter 3 is so switched as to minimize the residual signal S.sub.Z1 in accordance with the spectrum of the input digital signal S.sub.I.
In addition to the detection output signal Sp supplied to a noise filter 9 and a predictive filter 10 its side, a floating coefficient signal S.sub.F is also outputted from the maximum detector 8 on the basis of the maximum value of the residual signal S.sub.Z1 and then is supplied to a multiplier 11, whereby the residual signal S.sub.Z1 corrected to a predetermined dynamic range is inputted to a re-quantizer 12.
The re-quantizer 12 is fed with the residual signal S.sub.Z1 via an adder 13 and the multiplier 11, and after re-quantizing the residual signal S.sub.Z1, transmits the same to the receiving side.
At the receiving side, the transmitted signal S.sub.L1 is supplied through a transmission line L1 to the predictive filter 10 via a multiplier 18, which is inverse in characteristic to the aforementioned multiplier 11, and also via an adder 20, and then the output signal of the predictive filter 10 is fed back to the adder 20.
The frequency characteristic of the predictive filter 10 is switched similarly to the predictive filter 3 in accordance with the detection output signal Sp to consequently decode the transmitted signal S.sub.L1, hence realizing improved transmission of the input digital signal S.sub.I with a high efficiency enhanced by a value corresponding to the transmission of the residual signal S.sub.Z1 instead of the input digital signal S.sub.I.
In this stage of operation, the re-quantizer 12 is supplied with the signal S.sub.Z2 which is the difference between the input and output signals and is obtained by an adder 21, and such difference signal S.sub.Z2 is fed back to the adder 13 via a multiplier 22, which is inverse in characteristic to the multiplier 11, and also via a noise filter 9.
The noise filter 9 is so formed as to selectively switch the frequency characteristic in accordance with the detection output signal Sp.
When the predictive filter 3 is switched to the frequency characteristic represented by Eq. (1), the amount of feedback of a higher frequency component is reduced.
Therefore, in case the spectrum of the input digital signal S.sub.I is distributed in a higher frequency band, the noise generated during the re-quantization (hereinafter referred to as re-quantization noise) is emphasized in the higher frequency band when the signal is decoded, whereby the spectrum of the re-quantization noise is approximated to that of the input digital signal S.sub.I, and the signal-to-quantization noise ratio is improved by utilizing this aural masking effect.
Meanwhile, if any of the frequency characteristics represented by Eqs. (2), (3) and (4) is selected, the filter characteristic is switched to that of the predictive filter 3, so that when the signal is decoded, the spectrum of the re-quantization noise is flattened and the energy thereof is minimized.
In case the technique of noise shaping is applied to the input digital signal S.sub.I having a complicated spectrum as graphically shown in FIG. 2, it is rendered possible to improve the aural signal-to-quantization noise ratio most effectively by approximating the spectrum of the re-quantization noise S.sub.N to that of the input digital signal S.sub.I. (Hereinafter this technique will be referred to as optimum noise shaping.)
However, with the known circuit configuration of FIG. 1 where the predictive filter 3 is switched between four frequency characteristics and the noise filter 9 is selectively switched in conformity therewith, there exists a problem that optimum noise shaping cannot be applied to the input digital signal S.sub.I having various spectra, whereby sufficient enhancement is not achievable in the signal-to-quantization noise ratio.